The present invention concerns a reciprocating compressor, in particular a compressor having lubricated pistons.
Compressors having lubricated pistons currently in use include a crankcase within which the drive shaft of an engine is rotatably mounted, one or more cylinders fixed to the said crankcase, each provided with an associated cylindrical cavity in communication with the crankcase itself, and a head fixed to the top of each cylinder to close one end of the said cylindrical cavity with a fluid-tight seal.
The said compressors also include a piston associated with each cylinder, which piston is axially moveable within the cylindrical cavity and defines, together with its cylinder and the associated head, a fluid-tight chamber, and is moved by the said drive shaft by means of a crank and connecting rod assembly to vary the volume of the said fluid-tight chamber in such a way as to draw a fluid through an inlet valve on the head, to compress the said fluid and, finally, to expel the fluid from the variable-volume chamber through an outlet valve on the head.
In order to guarantee that the variable-volume chamber is fluid-tight, the piston has an annular sealing element or ring disposed coaxially with the axis of the piston and cylinder within a seat formed on a side wall of the piston itself, and capable of sliding on the side wall of the cylindrical cavity to prevent fluid leaking from the said chamber. This seat is formed close to the upper surface of the piston facing the cylinder head.
Furthermore, as the side walls of the cylindrical cavity are bathed with lubricating oil to facilitate the sliding of the piston in the associated cylinder, the piston has an annular oil-scraper ring disposed coaxially with the axis of the piston and cylinder within a seat formed in the side wall of the piston itself, capable of sliding over the side wall of the cylindrical cavity to prevent lubricating oil leaking into the fluid-tight chamber and damaging the head.
Currently, both the sealing ring and the oil-scraper ring are made from cast iron, hence the cylinder must be made from an extremely wear-resistant material, for example, cast iron, so that it is able to withstand the sliding without being damaged. In fact, by having to prevent the passage of the pressurised fluid into the crankcase, the cast iron sealing element exerts a significant pressure on the wall of the cylindrical cavity such that, if the cylinder is formed from an insufficiently hard material (such as, for example, aluminium), the walls themselves could become scored allowing either the pressurised fluid or the lubricating oil to pass along, irreparably damaging the compressor.
Unfortunately, making the cylinder from a hard and wear-resistant material such as cast iron is relatively expensive and leads to a significant increase in the production costs of compressors having lubricated pistons.